Iron aluminides, based on or near the Fe.sub.3 Al composition, are ordered intermetallic alloys that offer good oxidation resistance, excellent sulfidation resistance, and lower material cost than many stainless steels. Further, these materials conserve strategic elements such as chromium. They have a lower density than stainless steels and, therefore, offer a better strength-to-weight ratio. However, limited ductility at ambient temperature and a sharp drop in strength above 600.degree. C. have been major deterrents to the acceptance of the iron aluminide intermetallic alloys for use in structural applications.
The ductilization of iron aluminides has been the subject of research for more than 50 years. Typically, the maximum room temperature ductility obtained has been in the range of about 1 to 5%. In a commonly assigned patent application Ser. No. 319,771, filed Mar. 7, 1989, now U.S. Pat. No. 4,961,903 in the names of Claudette G. McKamey and Chain T. Liu, a composition based on Fe.sub.3 Al plus selected additions of chromium, molybdenum, niobium, zirconium, vanadium, boron, carbon, and yttrium is disclosed having a room temperature ductility up to about 10%. This application and the references cited therein are hereby incorporated by reference.
In a journal article entitled "An Environmental Effect as the Major Cause for Room Temperature Embrittlement in FeAl," by C. T. Liu, E. H. Lee and C. G. McKamey, Scripta Metallurgica, Vol. 23, pp. 875-880 (1989), a mechanism is proposed to explain the low ductility and brittle fracture problem in Fe.sub.3 Al and FeAl. The mechanism suggested by Liu et al involves the dissociation of water molecules in the environment by aluminum atoms on the surface of the alloy resulting in the formation of atomic hydrogen. The atomic hydrogen drives into the metal along cleavage plains during stressing, causing embrittlement. No solution to the problem is suggested.
In another journal article entitled "16 Percent Aluminum-Iron Alloy Cold Rolled in the Order-Disorder Temperature Range," by J. F. Nachman and W. J. Buehler, Journal of Applied Physics, Vol. 25, No. 3, pp. 307-313 (March 1954), discussion is provided regarding ductilization of iron aluminides. The author advocates reordering of the alloy by very slow cooling or by heating for prolonged periods in the temperature range of 450.degree. C. to 560.degree. C.